Silicone rubber has been used in a variety of fields as medical instruments, building materials, electric and electronic parts, automotive parts, and business machine parts because of its superior properties including physiological inertness (non-toxicity), weather resistance, durability, release properties, and heat resistance.
Various curing types of silicone rubber are available. Among them, a silicone rubber composition that cures through hydrosilylation reaction is known as the addition curing type. It readily cures simply by heating, leading to a very high production yield. Curing reaction entails no by-products. It is then especially useful for the production of medical tubes and building gaskets by extrusion molding.
In such applications of silicone rubber as very thin tubes and glass-supporting gaskets, the silicone rubber is required to have high hardness and high modulus, typically a hardness of at least 60 on JIS A hardness scale and a 50% modulus of at least 10 kgf/cm.sup.2. Such requirements can be met by one conventional approach of adding a relatively large amount of silica filler to a silicone rubber composition.
Although blending a relatively large amount of silica filler in a curable silicone rubber composition affords a silicone rubber composition having improved physical properties, the composition can be extruded with difficulty, detracting from the economy of extrusion molding. Blending of an extra amount of silica filler enhances the hardness and modulus of cured silicone rubber, but rather detracts from other physical properties, especially tear strength and compression set.
Japanese Patent Publication (JP-B) No. 18419/1972 discloses a silicone rubber composition comprising two types of organopolysiloxane having different viscosities and vinyl contents, silica, organohydrogenpolysiloxane, and a platinum catalyst, the composition yielding a silicone rubber having improved tear strength. Carrying out a follow-up test, we found that this silicone rubber had improved tear strength, but was insufficient in compression set and extrudability and had a low modulus. Japanese Patent Application Kokai (JP-A) No. 283265/1992 discloses a silicone rubber composition comprising a vinyl-containing organopolysiloxane and an organopolysiloxane resin copolymer having different viscosities, filler, an organic peroxide, and organohydrogenpolysiloxane, the composition yielding a silicone rubber having improved tear strengths and compression set. Since this silicone rubber composition is essentially vulcanized with organic peroxides, it can afford good tear strength and compression set when compression molded, but is not suitable for extrusion molding, for example, hot air vulcanization (HAV). If a low temperature decomposable organic peroxide is used to allow for HAV, the resulting silicone rubber is drastically reduced in tear strength and compression set. If a high temperature decomposable organic peroxide is used, the resulting silicone rubber composition is prone to compression molding to achieve acceptable tear strength and compression set, but not to HAV.